Goto is allowed only when the Shell is taking commands from a file. The
file is searched from the beginning for a line beginning with `:' followed by
one or more spaces followed by the label. If such a line is found, the
goto command returns. Since the read pointer in the command file points
to the line after the label, the effect is to cause the Shell to transfer to
the labelled line.

`:' is a do-nothing command that is ignored by the Shell and only
serves to place a label.